Laser robots of various types have been proposed, and an articulated industrial laser robot is disclosed in the pending International Patent Application PCT/JP90/00104 of the same Applicant as the present application. This laser robot obtains a laser beam emitted by a laser beam oscillator and led by conduit lines to the robot unit, and leads the laser beam through laser beam conduit lines arranged to run in the robot unit and an optical system for deflecting the laser beam to a laser beam projecting unit joined to an extremity of the robot arm of the robot unit toward a desired position. The robot unit of the laser robot comprises a plurality of movable components, such as a swivel post, robot arms and a robot wrist, and these movable components are controlled to direct the laser beam projecting unit toward the desired position. Accordingly, many drive motors, especially electric motors, are employed for driving those movable components of the robot unit for causing a turning, a swinging, and/or a telescopic motion thereof.
Nevertheless, since these drive motors actuate the movable components through mechanical transmission mechanisms, a thermal deformation of the respective mechanical transmission mechanisms caused by heat, i.e., Joule heat when the drive motors are electric motors, generated by the drive motors generates an error in the motion of the respective movable components.
Particularly, a swing mechanism disposed between the electric motor and the robot arm to drive the robot arm for an up-and-down swing motion about a horizontal axis in a vertical plane includes a precision ball-screw mechanism and a linkage, i.e., a mechanism comprising a ball-screw driven for rotation by an electric motor, and a ball-nut engaged with the ball screw and capable of transmitting a linear motion thereof through the linkage to the robot arm, to thereby swing the robot arm about a horizontal axis. Although the precision ball-screw mechanism has only a small amount of backlash, to be thus able to exhibit a high accuracy when controlling the robot arm, the axial length of the ball-screw is often changed when the ball screw mechanism is exposed to heat generated by the electric motor. Accordingly, the control of robot motions is made unstable by the heat generated during a transient period before the temperature of the ball-screw and the associated parts increased by the heat generated by the electric motor reaches a thermal equilibrium condition, after an electric power source connected to the robot unit is has been turned on to start the robot unit. This long transient period, however, significantly reduces the rate of operation of the laser robot.
To solve the problem attributable to the heat generated by the electric motor, another laser robot has been provided wherein a cooling fan is employed to provide forced cooling of the electric motor. Nevertheless, a cooling of the entire electric motor requires a cooling fan having a large cooling capacity, which increases the size of the cooling fan and unavoidably entails an increase in the manufacturing cost of the robot unit, in addition to inclusion of the fan per se.